Health Physics Questions and Answers - Question 455

Question 455: Part 50 license standard technical specifications define "Dose Equivalent I-131" as "...that concentration of I-131 (microcurie/gram) which alone would produce the same thyroid dose as the quantity and isotopic mixture of I-131, I-132, I-133, I-134, and I-135 actually present..." and "the thyroid dose conversion factors used for this calculation shall be those listed in NRC Regulatory Guide 1.109."

(a) After implementation of the revised 10 CFR Part 20, should licensees continue to use the Regulatory Guide 1.109 thyroid dose conversion factors or should they use the thyroid dose conversion factors in EPA Federal Guidance Report No. 11?

(b) Will this be addressed in NRC's forthcoming generic letter on changes to technical specifications related to the revised Part 20?


(a) Licensees must continue to use the thyroid dose conversion factors DCFs) that are referenced in their technical specifications (TS). A TS amendment would be needed to allow the use of other technically acceptable values. It should be noted that in the absence of such regulatory requirements, the NRC has allowed licensees to use sources of intake-to-dose conversion factors other than Regulatory Guide 1.109.

(b) The use of Federal Guidance Report No. 11 thyroid DCFs is not planned to be included in the generic letter on changes to power reactor technical specifications to incorporate the revised Part 20 but will be addressed in a forthcoming health physics position document (which will be made publicly available).

(Reference: Reactor Technical Specifications)

Page Last Reviewed/Updated Tuesday, December 12, 2017